Sound works in Blender, but no sound in the final render
I assume that you are using Windows if you are using xvid, but correct me if I'm wrong. I use a Mac, but this should transfer over in a manner of speaking.

On Macs, I can select the QuickTime video format. Once selected, there is a video codec option and an audio codec option. I generally select AAC as my audio codec. I would imagine that there is some format on Windows in Blender that works like QuickTime on Mac.